The present invention is directed to delivery of a glass stream for forming glass charges or gobs for glassware manufacture, and more particularly to an apparatus and method for delivering a so-called cased glass stream in which an inner or core glass is surrounded by an outer or casing glass.
It has heretofore been proposed to provide a cased glass stream for forming glassware having layered wall segments. U.S. Pat. Nos. 4,740,401 and 5,855,640, for example, disclose techniques for delivering such a cased glass stream, in which core glass from a first source is fed through at least one first orifice. At least one second orifice is vertically spaced beneath and aligned with the first orifice, and is surrounded by a chamber that communicates with the second orifice through a metering gap between the first and second orifices. A heated tube delivers casing glass from a second source to the chamber that surrounds the second orifice. Glass flows by force of gravity from the first and second sources through the first and second orifices in such a way that a cased glass stream emerges from the second orifice. This cased glass stream may then be sheared by conventional techniques to form individual cased glass gobs for delivery to conventional glassware forming machines.
Although the techniques disclosed in the noted patents address and overcome problems theretofore extant in the art, further improvements remain desirable. For example, a problem remains concerning uniformity of distribution of casing glass thickness around the circumference of the core glass stream. The noted patents teach that the dimensions of the metering gap around the orifices are chosen to provide uniform flow resistance to casing glass at all points circumferentially around the gap. In the preferred embodiments disclosed in the noted patents, the dimensions of each gap, both parallel and perpendicular to casing glass flow, are uniform around the gap. In implementation of this technique, it has been found that the casing glass can vary in thickness by a ratio of up to 2/1 around the circumference of the core glass stream. Computer modeling and temperature measurements indicate that this thickness uniformity is due at least in part to heat loss, and consequent increase in viscosity, in the casing glass flowing through the chamber to the metering gap.
U.S. application Ser. No. 08/822,629 filed Mar. 21, 1997 discloses a technique for indirectly addressing this heat loss problem by dimensioning the metering gap to be non-uniform around the aligned orifices. Specifically, the width of the metering gap in a direction perpendicular to casing glass flow through the metering gap (i.e., axially of the cased glass stream) has a wider dimension on the side of the metering gap opposite entry of the casing glass to the chamber surrounding the metering gap. By dimensioning the gap non-uniformly in this way, resistance to glass flow through the metering gap is tailored so as to compensate at least in part for the greater distance of glass travel through the chamber, and greater heat loss during travel, so as to improve uniformity in resistance to glass flow throughout the entire glass flow path through the chamber and the metering gap. Implementation of this technique has typically improved casing glass thickness uniformity to on the order of 1.1/1 to 1.2/1.
Although the technique disclosed in the noted copending application has improved casing glass thickness uniformity as indicated, further improvements remain desirable.
For example, a metering gap may be designed for a casing glass of a specific temperature and flow characteristic, but not be as well suited for a casing glass that possesses a different temperature or flow characteristic. It is therefore a general object of the present invention to provide an apparatus and method for delivering a cased glass stream of the character disclosed in the above-noted patents that obtain improved uniformity of casing glass thickness around the circumference of the cased glass stream. A more specific object of the present invention is to provide an apparatus and method of the described character in which loss of heat during casing glass flow through the casing glass chamber to the metering gap is directly addressed by adding heat to the casing glass during flow through the chamber, with the objective being to maintain constant glass temperature and therefore constant glass viscosity during flow through the casing glass chamber to and through the metering gap.
Apparatus for forming a cased glass stream having an inner core glass surrounded by an outer casing glass includes a first orifice for receiving core glass from a first source, and an orifice ring forming a second orifice vertically spaced beneath and aligned with the first orifice. A chamber surrounds the second orifice and communicates with the second orifice through a metering gap between the first and second orifices. Casing glass is delivered from a second source to the chamber, such that glass flows by gravity from the first and second sources through the orifices to form the cased glass stream. In accordance with one aspect of the present invention, a heater ring of refractory material is disposed beneath the orifice ring, and includes at least one electrical heater embedded in the refractory material of the heater ring. The electrical heater is thus disposed external to the casing glass chamber for adding heat to casing glass flowing through the chamber to the metering gap. The heater ring in the preferred embodiment of the invention underlies and is contoured for facing engagement with the entire undersurface of the orifice ring for transferring heat energy through the orifice ring to glass in the casing glass chamber.
In the preferred embodiment of the invention, at least one temperature sensor is embedded in the heater ring for sensing temperature of the heater ring, and a control circuit is responsive to the temperature sensor for selectively applying electrical energy to the heater. In the disclosed embodiment of the invention, there are a plurality of electrical heaters and a corresponding plurality of temperature sensors embedded in the heater ring. These coils may be connected in parallel to a common control circuit, or may be connected to corresponding separate temperature control circuits responsive to the associated sensors for separately applying electrical power to the several heaters. The metering gap may be of uniform dimension around the gap, or preferably is of contoured non-uniform dimension as disclosed in the above-noted copending application. Applying controlled heat energy to the casing glass as it flows through the chamber, combined with provision of a non-uniform casing glass metering gap, improves uniformity of casing glass thickness around the cased glass stream.